1. Field of the Invention
The present invention relates to detection of a position of a magnetic pole in a rotor included in a synchronous motor.
2. Description of the Related Art
In synchronous motors, a current is caused to flow through each of phase windings according to a position of a magnetic pole in a rotor for the purpose of generating a desired torque. An encoder or another sensor is used to detect the position of a magnetic pole in the rotor so that a current will flow in the direction of an appropriate excitation phase. This necessitates the employment of a sensor that is sensitive to an absolute value permitting detection of the position of a magnetic pole or the work of aligning a sensor with the position of a magnetic pole. In efforts to obviate the necessity of the employment of the sensor and the work of alignment, detection of the position of a magnetic pole is performed at the time of starting a motor.
Various methods have been proposed as methods of detecting a position of a magnetic pole without a sensor. According to methods including, for example, a method disclosed in Japanese Unexamined Patent Application Publication No. 2000-312493 (first patent document), certain voltages of positive and negative polarities that are regarded as vectors are applied to a permanent-magnet motor with the angle between the vectors varied at regular intervals. Currents flowing through the motor in responsive to the positive and negative voltages are detected. The positions of the terminal points of the vectors at which the currents have the largest difference are detected as the positions of magnetic poles.
According to methods including, for example, a method disclosed in Japanese Unexamined Patent Application Publication No. 2002-78391 (second patent document), a microscopic voltage transient is produced in a salient-pole type motor, and a position of a magnetic pole is inferred from the microscopic voltage transient and a change rate of a component of a current flowing through the motor.
According to methods including, for example, a method disclosed in Japanese Unexamined Patent Application Publication No. 2002-165483 (third patent document), a high-frequency voltage is applied to a salient-pole type motor. A high-frequency current whose frequency is identical to the frequency of the high-frequency voltage is sampled from a current flowing through an armature. The position of a rotor is inferred from the high-frequency current. A positive or negative offset is added to the inferred position of a rotor according to the polarity of the high-frequency current detected immediately after the motor is started, whereby an initial value of the inferred position of a rotor is calculated.
According to methods including, for example, a method disclosed in Japanese Unexamined Patent Application Publication No. 2003-52193 (fourth patent document), a high-frequency voltage is applied to a salient-pole type motor, and a position of a magnetic pole is detected based on a detected high-frequency current.
According to methods including, for example, a method disclosed in Japanese Patent No. 3408368 (fifth patent document), a short-period current is caused to flow in the direction of a predetermined excitation phase in a stator. Based on the direction of a motion made by a rotor at that time, the excitation phase is changed and the short-period current is caused to flow. This process is repeated in order to detect the position of a magnetic pole.
In terms of a structure, a rotor included in a permanent-magnet synchronous motor falls broadly into a surface permanent magnet (SPM) type having a permanent magnet bonded to the surface of a rotor and an interior permanent magnet (IPM) type having a permanent magnet embedded in a rotor. In general, the SPM motor is of a non-salient pole type in which a d-axis inductance and a q-axis inductance offered by windings on an armature are equal to each other, while the IPM motor is of a salient-pole type in which the d-axis and q-axis inductances are different from each other. The methods described in the first to fourth patent documents for detecting a position of a magnetic pole by applying a voltage to a motor and detecting a current that flows through an armature can be adapted to the salient-pole type motor but cannot be adapted to the non-salient pole type SPM motor.
On the other hand, a method described in the fifth patent document can be adapted to any synchronous motor irrespective of whether the synchronous motor is of the salient-pole type or non-salient pole type. However, the method requires some motion of a rotor. If the rotor makes no motion because of large friction of a machine, a position of a magnetic pole cannot be detected correctly.
The present applicant has proposed a method of detecting a position of a magnetic pole without the motion of a rotor in Patent Application No. 2003-362379. However, the method utilizes the electric characteristic of the salient-pole type and magnetic saturation and cannot therefore be adapted to the non-salient pole type SPM motor.